The Conservatives are committed to the tax break - which is one of the Tories totemic policies.
Labour have already pledged they would abolish what they say is a "perverse and unfair" allowance - potentially making any coalition negotiations with the Lib Dems easier.
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The party say they would spend the money in England by expanding free childcare.
In Scotland, where childcare is devolved, the move would see cash transferred north of the border for Scottish ministers to spend.
The plan is a key plank of the Lib Dems "pre-manifesto" of almost 300 policies to be unveiled by Deputy Prime Minister Nick Clegg later today.
The 80-page document, "Stronger Economy, Fairer Society", will form the basis of the Lib Dem manifesto in next May's general election and will be voted on by delegates at the party's annual conference in Glasgow next month.
A senior Lib Dem source described the marriage allowance as a "penalty on unmarried couples", including war widows and abandoned wives.
The Tories forced through the tax breaks this Parliament in the face of opposition from their Lib Dem partners.
But it will not come into effect until next April.